By associating closely with others to form a group, an animal can benefit from a number of advantages including reducedrisk of predation, amelioration of environmental conditions, and increased reproductive success, but at the price of reducedresources. Although made up of individual members, an aggregation often displays novel effects that do not manifest at thelevel of the individual organism. Here we show that very simple behaviour in intertidal mussels shows new effects in denseaggregations but not in isolated individuals. Perna perna and Mytilus galloprovincialis are gaping (periodic valve movementduring emersion) and non-gaping mussels respectively. P. perna gaping behaviour had no effect on body temperatures ofisolated individuals, while it led to increased humidity and decreased temperatures in dense groups (beds). Gaping resultedin cooler body temperatures for P. perna than M. galloprovincialis when in aggregations, while solitary individuals exhibitedthe highest temperatures. Gradients of increasing body temperature were detected from the center to edges of beds, butM. galloprovincialis at the edge had the same temperature as isolated individuals. Furthermore, a field study showed thatduring periods of severe heat stress, mortality rates of mussels within beds of the gaping P. perna were lower than those ofisolated individuals or within beds of M. galloprovincialis, highlighting the determinant role of gaping on fitness and groupfunctioning. We demonstrate that new effects of very simple individual behaviour lead to amelioration of abiotic conditionsat the aggregation level and that these effects increase mussel resistance to thermal stress.